Following selective coordination best practices
Design engineers must coordinate electrical systems so that the protective device closest to the fault opens first, and quickly enough, to prevent the upstream devices from tripping.
- Demonstrate when selective coordination is required.
- List the various codes and standards that define selective coordination.
- Compare protective devices.
Most people have experienced some type of electrical power loss. Some power losses are localized, involving only one—or perhaps several—circuits. Other power losses are spread over a wide area and involve an entire facility or several facilities. Regardless of how robust an electrical system may be, electrical faults happen. It's in the interest of everyone that the fault affects only a limited area of the electrical system, especially where life safety is concerned. That is why NFPA 70: National Electrical Code (NEC) requires selective coordination of protective devices, such as circuit breakers and fuses, in emergency systems.
What is selective coordination?
Although the definition has changed slightly over the years, NEC 2014—the most recent version of the code—defines selective coordination as: "Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices (OCPDs) and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of OCPD opening times associated with those overcurrents."
In other words, the electrical engineer must carefully consider OCPDs so that the protective device closest to the fault opens first and quickly enough to prevent the upstream devices from tripping (see Figure 1). In Figure 1, the fault occurs downstream near the load. Selective coordination requires that only "circuit breaker one" clear the fault, allowing the other breakers in the switchboard identified as "NSG" to continue normal operation. The other circuit breakers in the switchgear would not see the fault; as a result, the rest of the loads would continue to be fed without interruption.
NFPA 110-2016: Standard for Emergency and Standby Systems also requires coordination in Article 6.5: "The OCPDs in the emergency power supply system shall be coordinated to optimize selective tripping of the circuit OCPDs when a short circuit occurs."
It seems very reasonable to have the protective devices selectively coordinated. Nobody likes power outages. The problem is that everything comes at a cost, and selective coordination makes for a more expensive electrical system. Because NEC is primarily interested in the safety of humans, it requires selective coordination only in systems that directly affect life safety. To what extent one needs to coordinate the protective devices depends on the authority having jurisdiction. In some locations, the requirement is to coordinate to 0.01 second and other locations to 0.1 second.
As with everything else, NEC turned attention to selective coordination as a matter of life safety. The coordination requirement was first introduced in 1993 in relation to elevators (Article 620: Elevators, Dumbwaiters, Escalators, Moving Walks, Platform Lifts, and Stairway Chairlifts). In 2005, the requirement was expanded to include emergency systems (Article 700), legally required standby systems (Article 701), and health care facilities (Article 517). The definition of selective coordination was also added in 2005 to better define the requirement. In 2008, Article 708: Critical Operations Power Systems was added to the requirements. Now, the requirement for selective coordination includes Articles 695: Fire Pumps and Article 645: Information Technology Equipment, as well.
When is selective coordination required?
Selective coordination is not required everywhere. It is only required where power continuity is critical to human life or when an interruption of power can cause hazardous conditions.
NEC Article 700.28 states: "Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side OCPDs. Selective coordination shall be selected by a licensed professional engineer or other qualified persons engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system."
It makes for a lively discussion when someone says selective coordination is only required on the emergency-power-source side of the electrical system. The reasoning behind this line of thought is that if the normal power is interrupted, the emergency system will take over, thereby providing the necessary power. The problem is that if the normal power protective device is not coordinated with the downstream protective device, the upstream device could trip first. The automatic-transfer device will ask the emergency system to energize and transfer over when the emergency power is established. In this case, the emergency system will be feeding a fault, which could force the whole system out of service.
NEC Article 701.27 states: "Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply-side OCPDs." The protective device upstream of the automatic-transfer device is also part of these "supply-side OCPDs." As such, it should be coordinated with the emergency system protective devices downstream of the automatic-transfer device (see Figure 2).